A loudspeaker vibrating in isolation produces very little sound. The reason for this is that the waves formed in the front and back of the loudspeaker can effectively cancel each other out. When the loudspeaker's cone is thrust forward, a high-pressure compression is formed in the front and a low-pressure rarefaction is formed in the back of the cone. If the wavelength of the sound is large compared to the dimensions of the loudspeaker, an air flow will be set up between the high-pressure and low-pressure regions with the result that the sound intensity is substantially reduced.
To prevent such reduction in sound intensity, a loudspeaker may be mounted in a baffle. The baffle prevents the air in front from communicating with the air in back of the speaker. A baffle is effective as long as the resulting path length between the front and back of the speaker is greater than the wavelength of the sound. In other words, the time required for a disturbance to travel from the front to the back must be greater than one period of the cone's motion.
Loudspeakers, however, are not normally mounted in baffles. Typically, loudspeakers are mounted in an enclosure. While such an arrangement prevents the transport of air from the front to the back of the loudspeaker, other problems arise that are related to low frequency audio reproduction. With respect to low frequency audio (1-150 Hertz), the human ear cannot generally detect audio signals below approximately 20 Hz. Yet, the vibrating sensations felt by audio signals below 20 Hz that are typically present during a live performance enhance the listening experience. However, even the best low frequency speaker systems, or sub-woofers as they are known, are only able to efficiently reproduce low frequency signals down to about 15 Hz and generally require a great deal of power to do so.